This invention relates generally to an electrophotographic printing machine and more particularly concerns a method and apparatus which uses analog output information of a document reproduction device to provide real-time adjustment of a digitized image in order to adjust density output of printed color or monochrome documents.
By way of background, digital reproduction, transfer or display of original images on image output terminals begins by creating a digital representation of an original image. Commonly, the digital representation becomes a two-tone microstructure otherwise known as a bitmap. In conventional halftoning, multiple gray levels or gray densities in the original image are reproduced by varying the amplitude within a fixed spatial frequency of halftone microstructures (or halftone cells/dots). Continuous tone images or image portions are typically represented in binary format by creating halftone cells or dots, where each cell represents a gray level density within an area of picture elements pixels).
Methods of halftone digital image processing, encompassing the process steps of scanning or image acquisition through printing or display are known. In general, digital image output terminals (e.g. printers) are capable of creating spots within an area with a predetermined resolution (dots per unit area). In scanners, a single xe2x80x9cspotxe2x80x9d describes a multi-bit density response. Typically, input scanners may acquire density information at 256 levels of gray to describe a spot or pixel. By contrast, output terminals generally have two or another relatively small number of levels to reproduce image information.
In printing systems maintaining stability and accuracy as to the amount of a marking material (e.g. toner or ink) being applied to a print surface is a major concern. Specifically it is known that due to varying conditions the amount of marking material (e.g. cyan, magenta, yellow and black toner or ink) will fluctuate from a predetermined value. For example, humidity, toner or ink age, machine calibration, toner or ink quality can all cause the amount of marking material applied to a print surface to vary.
Therefore, in color and black and white copiers or printers, a common technique for monitoring the quality of documents is to artificially create test patches of predetermined desired densities. The actual density of the toner of the test patches can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In the case of xerographic devices, the surface that is typically of most interest in determining the density of printing material is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently, developed by causing toner particles to adhere to areas that are charged in a particular way. In such a case, the density detector for determining the density of toner on the test patch, which is often referred to as a xe2x80x9cdensitometerxe2x80x9d, is disposed along the path of the photoreceptor directly downstream of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system to charge or discharge, as necessary, the surface at a predetermined location.
The test patches are moved past the development unit and the toner particles within the development unit are caused to electrostatically adhere to the test patches. The denser the toner on the test patches, the darker the test patches will appear in optical testing. The developed test patches are moved past a density detector disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch. Xerographic test patches are traditionally printed in the inter-document zones on the photoreceptor. They are used to measure the deposition of toner on paper to measure and control the toner reproduction curve (TRC). A common method of process control involves scheduling solid area, uniform halftones or background in test patches. High quality printers will often contain many test patches.
Depiction of a process whereby a printing machine maintains output print density is illustrated in FIG. 1. Shown is an image path A for a printing machine where a page description module (which implements a page description language (PDL) such as, but not limited to, PostScript) 10 forwards image information to Raster Image Processor module (RIP) 12. The RIP generates a rasterized image (in this example, a 600xc3x97600xc3x978 image) 14. Through this procedure a contone image (e.g. having cyan, magenta, yellow and black) is described in a gray level format. Contone Rendering Module (CRM) 16 receives rasterized image 14 and performs a halftoning operation 16a of the rasterized image 14 in accordance with a predetermined tone reproduction curve (TRC) 16b. The CRM 16 then generates a rasterized binary (bitrmapped) image having a high addressability factor (e.g. 600xc3x974,800xc3x971) 18. This binary image is provided to Raster Output Scanner (ROS) 20 which in turn generates photoreceptor image 22. Using photoreceptor image 22, normal known xerographic operations are undertaken for the generation of a color print.
Consistent tone reproduction is a high priority in color production markets. Even slight (e.g. 2xcex94Ecmc) color changes within a job can be objectionable to a customer. Within the architecture described in FIG. 1, constant tone reproduction for printed outputs over time is maintained by feeding real-time (inter-document zone) xerographic density information to CRM 16 of the print engine. CRM 16 applies an appropriate TRC 16b to each contone image and then the image is halftoned to a binary high-addressable image space understood by ROS 20.
Data concerning the xerographic density of patches on photoreceptor image 22 are provided to the CRM 16 by process control feedback 24. In order to maintain a stable printing operation despite the fact that the print engine output may be varying, the tone reproduction curve (TRC) 16b is applied immediately before the halftoner operation 16a. Thus, if the signal from process control feedback 24 indicates the xerographic density values are off a nominal amount, TRC 16b is changed in front of halftoner operation 16a in order to provide desired print outputs. For example, if the printer is determined to be printing overly high yellow amounts of toner, the tone reproduction curve will be adjusted slightly down so that the yellow toner amount requested is decreased, thereby maintaining a stable printing output.
However, under the foregoing process there exists a need to be able to update TRC 16b on the print engine CRM 16 due to the possibility of a customer job not completely printing before the xerographic density state has changed. In a machine implementing image path B, if the xerographic density state changes during the time a job is printing, this system which undertakes an original halftoning process at RIP module 12, is forced to re-RIP the job in progress. The re-RIP would be done with the new xerographic density information. Due to this productivity impact, as well as the cost involved in providing a machine having such capabilities, quality benefits provided by a RIP that halftones is not fully attained.
A further drawback of the foregoing system is that the halftone operation must be accomplished in a real-time operation, which in turn requires implementation of halftoner 16a in CRM 16 which increases the cost of such a printing machine.
Therefore, it has been determined desirable to provide a method and apparatus that effectively adjusts the ROS image TRC after the image is in a binary printable space, and where the real-time adjustment may be made in reaction to a change in the xerographic density state.
A concept of the present invention is to perform a RIP to a binary (bitmapped) halftoned image (e.g. 600xc3x974,800xc3x971). Then the information received from xerographic density detectors monitoring the photoreceptor image allows for this xerographic density information to be used to change binary bit patterns of the bitmapped image instead of attempting to change a contone pattern. One manner of performing this change is through the application of a template matching operation.
Using the procedure of the present invention, a xerographic density detector sends a signal which indicates the status of xerographic density of test patches on the photoreceptor. The system is designed to monitor each of the individual colors (e.g. cyan, magenta, yellow and black) which are represented by the test patches. When a particular color is determined to be running at a level above or below a predetermined bit density value, information obtained by a scanning operation is reviewed. The information corresponds to at least a portion of an image which is to be printed. When a specific bit pattern or state is detected a template matching process is undertaken, wherein a determination is made as to whether a template matching the scanned image bit pattern exists in a template storage. When such a template is found to exist, the appropriate template is used in place of a corresponding scanned image area in order to counteract the faulty operation of the xerographic printer. In this process the system looks at one halftone dot and indicates that the dot is either too large or too small for the amount of a predetermined acceptable bit density. Therefore, to adjust the binary bit pattern, an appropriate template with either larger or smaller dot patterns are substituted. The final outcome therefore, results in an amount of toner or ink being applied to the document such that the desired output is achieved.